1. Field of the Invention
The present invention relates generally to an apparatus and method for providing a packet data service in a communication system, and in particular, to a packet data transmitting/receiving apparatus and method for detecting a packet data transmission/reception scheme.
2. Description of the Related Art
While a typical mobile communication system supports voice service only, user needs and the development of mobile communication technology have introduced a mobile communication system that additionally supports data service.
In a mobile communication system supporting multimedia service including voice and data services, a plurality of users receive voice service in the same frequency band and data service in TDM (Time Division Multiplexing) or TDM/CDM (Time Division Multiplexing/Code Division Multiplexing). One code applies to one slot assigned to a particular user in TDM, whereas a predetermined time is divided into a plurality of slots and a plurality of users to which unique orthogonal codes (e.g., Walsh codes) are assigned for identification use one time slot simultaneously in TDM/CDM.
An F-PDCH (Forward Packet Data Channel) is transmitted in a different manner depending on whether TDM or TDM/CDM is used. In TDM, packet data is delivered to a single user for a predetermined time period. Basically, all available Walsh codes are used as spreading codes for the F-PDCH.
In TDM/CDM, the F-PDCH delivers packet data to two or more users for a predetermined time period. Selected Walsh functions are used to spread data for the users to identify their data. Hence, information about Walsh functions used for the respective users is transmitted to the users on a PDCCH (Packet Data Control Channels).
The PDCH delivers packet data on a PLP (Physical Layer Packet) basis. PLP length is variable. To efficiently receive packet data on the PDCH, a control information frame (preamble) containing necessary control information is transmitted on the PDCCH (e.g., a secondary PDCCH: SPDCCH).
If the PDCH is transmitted in TDM/CDM, that is, if packet data is transmitted to a plurality of users simultaneously in one or more slots in CDM, the amount of control information in a preamble is variable according to the number of the users. The length of the preamble depends on the length of the packet data. In other words, once the receiver estimates the length of the preamble, the receiver can determine the packet data length. The preamble length is estimated by BSD (Blind Slot Detection).
FIG. 1 is a block diagram illustrating a conventional PDCCH transmitter supporting only TDM for PDCH transmission. Referring to FIG. 1, it is assumed that control information transmitted on the PDCCH, a PDCCH input sequence is 13 bits for N slots (N is 1, 2 or 4), but is not limited to 13 bits. The slot length of the PDCCH input sequence varies according to the slot length of packet data, but is determined irrespective of the length of a preamble. For example, if the packet data length is 1, 2, 4, or 8 slots, the preamble has a corresponding length. If the packet data is transmitted in 1 slot, a 1-slot preamble is transmitted. If the packet data is transmitted in 2 slots, the preamble is also 2 slots. If the packet data occupies 4 slots, the preamble is also transmitted in 4 slots. However, if the packet data is 8 slots, a 4-slot preamble is transmitted to avoid an excess increase in the preamble length.
In operation, a CRC adder 101 adds eight CRC bits to the 13-bit PDCCH input sequence. As the number of CRC bits increases, transmission error detection performance increases. Yet, power efficiency decreases. Thus eight CRC bits are usually used.
A tail bit adder 102 adds eight tail bits with all 0s to the CRC-attached control information received from the CRC adder 101. A convolutional encoder 103 encodes the output of the tail bit adder 102 at a code rate (R) of ½ for N=1, and at a code rate R of ¼ for N=2 or 4. Hereinafter, N indicates the slot length of control information on the PDCCH. A preamble has twice as many symbols after R=¼ encoding than after R=½ encoding, and a four-slot preamble needs to have twice as many symbols as a two-slot preamble. Therefore, a symbol repeater 104 repeats the four-slot preamble correspondingly. That is, the symbol repeater 104 simply outputs input data or repeats it once or three times, according to the slot length of the data. As a result, the symbol repeater 104 outputs 58N (N is 1, 2, or 4) symbols.
A puncturer 105 punctures 10N symbols in the output of the symbol repeater 104 and outputs 48N symbols to minimize performance degradation and match a desired data rate. An interleaver 106 interleaves the punctured symbols to permute the sequence of the symbols and thus reduce burst error rate. A BRI (Bit Reverse Interleaver) can be used as the interleaver 106. The BRI maximizes the space between adjacent symbols. After interleaving, the first half of the symbol sequence has even-numbered symbols and the second half, odd-numbered symbols. A modulator 107 modulates the interleaved symbols in a modulation scheme such as QPSK (Quadrature Phase Shift Keying).
FIG. 2 is a block diagram illustrating a conventional PDCCH receiver. Referring to FIG. 2, to determine the number of slots in which packet data is received from the transmitter, the receiver includes first to fourth reception units 210 to 240. The slot length of the packet data is determined through the CRC-check of the received data in the first to fourth reception units 210 to 240. The first reception unit 210 processes a one-slot preamble for one-slot packet data, the second reception unit 220 processes a two-slot preamble for two-slot packet data, the third reception unit 230 processes a four-slot preamble for four-slot packet data, and the fourth reception unit 240 processes a four-slot preamble for eight-slot packet data.
In each reception unit, a deinterleaver deinterleaves the received packet data according to a corresponding slot length and a depuncturer depunctures the deinterleaved symbols according to the slot length. In the third and fourth reception units 230 and 240, combiners 235 and 245 combine two consecutive symbols corresponding to the operation of the symbol repeater 104 illustrated in FIG. 1.
A convolutional decoder 216 decodes the depunctured symbols received from the depuncturer 214 at a code rate of ½, a convolutional decoder 226 decodes the depunctured symbols received from the depuncturer 224 at a code rate of ¼, and convolutional decoders 236 and 246 decode the combined symbols received from the combiners 235 and 245 at a code rate of ¼.
CRC checkers 218 to 248 CRC-check the decoded symbols using predetermined initial values. A packet length determiner 250 determines the slot length of the packet data according to the reception results received from the reception units 210 to 240. The reception units 210 to 240 may be separated physically or integrated into one reception unit with different reception parameters.
FIG. 3 is a diagram illustrating slot detection timing and slot lengths when the receiver illustrated in FIG. 2 receives the PDCCH. As noted from FIG. 3, the first to fourth reception units 210 to 240 operate for N=1, 2, or 4.
As described above, the conventional PDCCH supports only TDM transmission of the PDCH. Therefore, there is a need for a novel PDCCH structure to support TDM/CDM transmission of the PDCH.